Danfoss VLT 6000 (Legacy Product) User guide

Brand: Danfoss

Model: VLT 6000 (Legacy Product)

Type: User guide

Danfoss VLT 6000 (Legacy Product) is a versatile and efficient frequency converter designed to control AC motors in a wide range of industrial applications. With its advanced features and robust construction, it offers precise speed control, energy savings, and reliable operation. It is commonly used in variable torque applications such as fans, pumps, and compressors, and can also be used for constant torque applications with derating.

VLT

6000 HVAC



MN.60.D1.02 - VLT is a registered Danfoss trademark

CHILLER

Fig. 1 - Traditional Primary/Secondary design

Pressure

Flow

100%

75%

86%

84%

Efficiency

curves

Design flow Flow 1

Impeller

curves

Pressure

Flow 1

Design - P2

Design

flow

■■

■ The Design

In Chilled Water systems, the Primary loop or

production loop, consists of pumps sized to handle

the chillers designed flow rate at a discharge pres-

sure just high enough to circulate the water through

the chiller and the rest of the primary piping loop.

This loop should be as small as possible, just large

enough to allow the secondary system to be

attached. This minimizes the resistance of the

primary loop and therefore the energy consumption

of the non-regulated constant flow pumps.

The primary pump flow is traditionally controlled by

throttling valves or balance valves on the discharge

of the pumps, figure 1. The pumps are often

oversized due to safety margin in the designs. By

adding losses to the pumping circuit with the

throttling valves, the proper design flow rate can be

established, figure 2 (moving from flow 1 to Design

flow).

Another method used is to trim the pumps impeller.

Once the system is operating, the balancing

contractor can determine the actual pressure drop

in the primary loop.

Once the actual pressure requirement of the pump

is established, the pumps impeller can be removed,

trimmed to the proper diameter, rebalanced, and

reinstalled in the pump.

Decreasing the diameter of the pumps impeller

reduces both the capacity and pressure of the

pump as desired, but also has an impact on the

pumps efficiency, figure 3.

■■

■ The Application

Primary pumps in a primary/secondary pumping

system, such as shown in Figure 1, can be used to

maintain a constant flow through devices that

encounter operation or control difficulties when

exposed to variable flow. The primary/secondary

pumping technique decouples the primary

production loop from the secondary distribution

loop. This allows devices such as chillers to obtain

constant design flow and operate properly while

allowing the rest of the system to vary in flow.

Fig. 3 - Impeller trimming

Fig. 2 - Throttling valve

As the evaporator flow rate decreases in a chiller,

the chilled water begins to become over-chilled. As

this happens, the chiller attempts to decrease its

cooling capacity. If the flow rate drops far enough,

or too quickly, the chiller cannot shed its load

sufficiently and the chillers low evaporator

temperature safety trips the chiller requiring a

manual reset. This situation is common in large

installations especially when two or more chillers in

parallel are installed if primary/secondary pumping is

not utilized.

Primary system Secondary system

VLT

6000 HVAC

MN.60.D1.02 - VLT is a registered Danfoss trademark

CHILLER

Flowmeter

■■

■ The new standard:

Depending on the size of the system and the size of

the primary loop, the energy consumption of the

primary loop can become substantial.

A VLT frequency converter can be added to the

primary system, to replace the throttling valve and/or

trimming of the impellers, leading to reduced

operating expenses (fig. 4). Two control methods are

common:

The first method uses a flow meter. Because the

desired flow rate is known and is constant, a flow

meter can be installed at the discharge of each

chiller can be used to control the pump directly.

Using the built-in PID controller, the VLT frequency

converter will always maintain the appropriate flow

rate, even compensating for the changing resistance

in the primary piping loop as chillers and their pumps

are staged on and off.

Fig. 4

The VLT frequency converter solution

The other method is local speed determination. The

operator simple decreases the output frequency

until the design flow rate is achieved.

Using a VLT frequency converter to decrease the

pumps speed is very similar to trimming the pumps

impeller, except it doesnt require any labor and the

pumps efficiency remains higher, figure 5. The

balancing contractor simply decreases the speed of

the pump until the proper flow rate is achieved and

leaves the speed fixed. The pump will operate at

this speed any time the chiller is staged on.

Because the primary loop doesnt have control

valves or other devices that can cause the system

curve to change and the variance due to staging

pumps and chillers on and off is usually small, this

fixed speed will remain appropriate. In the event the

flow rate needs to be increased later in the systems

life, the VLT frequen-cy converter can simply

increase the pumps speed instead of requiring a

new pump impeller.

Fig. 5

Pump efficiencies with variable speed

(Figure courtesy of ITT Bell & Gossett)

VLT

Flow

Figure 5 shows that the pumps efficiency remains

constant as the speed is reduced to obtain the

design flow. This differs from the results of the

trimming the impeller where the efficiency de-

creases (fig. 3).

VLT

6000 HVAC

MN.60.D1.02 - VLT is a registered Danfoss trademark

■■

■ Energy saving calculation example

In the following calculation example, a 30 kW

primary pump´s operation cost with a VLT frequency

converter is compared to the same pumps opera-

tion cost with a throttling valve.

For the purposes of a comparison, the pumps

original design discharge pressure is assumed to be

15% overheaded.

■■

■ Annual operation load profile

Since primary pumps by definition are constant flow,

the flow rate is always at 100%. What the profile

doesnt show is the discharge pressure required.

Example:

A 30 kW primary pump is operating by a constant

flow at 100% for 24 hours every day.

Total annual operating hours is: 24h x 365 days = 8760 hours.

Energy consumption per year: 30 kW x 8760h = 262800 kWh

Energy saved by using a VLT: 15% of 262800 kWh = 39420 kWh

Annual money saved by using a VLT: 39420 kWh x USS 0,10 = USS 3942

Simple payback: ≈ 1.5-2.5 years

It is normal for pumps to be overheaded by 15% or

more to compensate for possible installation varia-

tions and other safety calculations. For constant

flow applications, this overpressurization causes a

proportional waste of energy. Using a VLT frequency

converter to balance the system instead of throttling

valves allows for the elimination of the otherwise

wasted energy.

■■

■ Sensor Type And Placement

For Primary Pumping systems, the sensor location is

not as critical since a flowmeter is not really required,

but when used, proper location will avoid any

operational difiiculties. The proper location for a flow

meter is at the discharge or inlet of each chiller,

assuming each chiller has a dedicated pump and

VLT frequency converter.

This allows the VLT frequency converter system to

guarantee the design flow rate through each chiller

and take advantage of the decreased resistance in

the piping network as parallel chillers are destaged.

An example schematic showing the sensor

placement is shown in Fig 4.

Differential pressure sensors can also be located

across each chiller because differential pressure

across a non modulating load also measures flow.

Since flow is the critical parameter however, it is

recommended that a flow meter be utilized.

Drive price

Energy savings

VLT

6000 HVAC

MN.60.D1.02 - VLT is a registered Danfoss trademark

■■

■ Comparison of installation and maintenance

costs

Aside from the potential energy savings,the cost of

using a VLT frequency converter can be partially

paid for by the savings on installation and main-

tenance costs. The traditional system not only

requires the throttling valves and balancing valves,

but also needs at a minimum: 6-wire motor cable, a

softstarter, and power factor correction capacitors

(PFCC´s). When opting to trim the pumps impeller,

system down-time and a considerable amount of

labor is also saved.

Maintenance is limited to a minimum, and instal-

lation costs and space can be saved. The VLT

frequency converters IP 54 enclosure allows it to be

mounted in the mechanical equipement room close

to the pumps on the wall.

A smart design combined with the falling costs of

electronic equipment and increasing costs in labor

have brought the investments for most regulation

methods to a similar level. The comparison of the

energy consumption, installation costs, and de-

creased maintenance costs show why the utilization

of VLT frequency converters are the best choice.

Utilizing the Danfoss VLT frequency converter

makes the valves, soft-starters, power factor

corrections, and extensive cablework unnecessary.

Manual speed adjustment or a simple voltage

(0...10V) or current (0/4...20 mA) control signal and

flow meter is sufficient to vary the flow.

■ ■

■ True pay-back calculation:

Cost of drive + flow meter - (throttling valves + starter + wiring + PFCC´s) - saving in commissioning time

Energy savings + annual maintenance savings

≈ 0 year ± 0.5 year

Danfoss VLT 6000 (Legacy Product) User guide

Danfoss VLT 6000 (Legacy Product) User guide

Danfoss VLT 6000 (Legacy Product) User guide

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